Mexican immigration is probably the most controversial demographic issue facing policy makers and citizens in the United States today. The proposal seeks continued support for the Mexican Migration Project (MMP) to provide high-quality public use data on the characteristics and behavior of documented and undocumented Mexican migrants to the United States. To date the project has surveyed 118 communities, yielding a database of 128,940 persons enumerated in 19,726 households, which together contain 21,988 current and former migrants to the United States, 13,793 of whom were undocumented at the time of the survey. Data will be disseminated from the MMP website, which currently has 1,508 registered users and receives an average of 5,079 hits per week. In the coming funding cycle, we propose to add 25 new community samples containing 5,500 households and roughly 34,650 individuals to the database, paying special attention communities in migrant-sending states that are not currently represented in the MMP sample. In addition, we propose to add a module of basic health questions to the survey instrument to enhance its value to data users and policy makers, and to add selected environmental indicators to the list of community-level variables available to users. PUBLIC HEALTH RELEVANCE: Mexican migration to the United States is among the most important contributors to U.S. population dynamics. It influences the growth and distribution directly through documented and undocumented migration, but also indirectly through its secondary effects on mortality and fertility. This project seeks to provide demographers and policy makers with accurate and reliable data on the characteristics and behavior of undocumented Mexican migrants to the United States.